1. Field Of The Invention
The present invention generally relates to a solid state primary cell, and more particularly, to maintenance of discharge performance for alkali metal-halogen cells with simplified methods of making the same. The cells are preferably housed inside of stainless steel cases and the improvements are realized by heat treating the inner surface of the case in an oxidizing atmosphere according to the present invention prior to introduction of the cell active components.
2. Prior Art
The condition of the internal surface of the stainless steel casings employed in lithium/halogen cells, particularly lithium/iodine cells, has been determined to be critical to cell electrical performance. In many case-positive cell designs, the casing inner surface or wall serves as the cathodic current collector, facilitating electron transfer during discharge. Voltage, impedance and delivered capacity at low current drains can all be adversely affected by contaminants which may be present on casings in their condition as received from suppliers.
Conventional practice in the preparation of the cell casing used in construction of alkali metal-halogen cells is to first acid treat the case to remove surface contamination. With a stainless steel case, acid bright dipping (known as Diversey.RTM.) is very effective, but it is a time-consuming and increasingly expensive process. Approximately three percent of the weight of each case is removed, dissolved in the acid along with any surface contaminants also present. Thus, bath life becomes limited due to the increasing concentration of metal ions in the costly, strong acid solution with use. In addition, the total process requires two active pretreatment steps, along with copious rinsing and final solvent drying. The acid dipping process is labor-intensive and time consuming. Finally, environmentally sound disposal of the heavy metal-laden spent acid and rinse wastes is becoming prohibitively expensive.
There is therefore a need for a casing, preferably a stainless steel case, for an alkali metal-halogen cell wherein the casing inner surface is essentially free of contaminants to ensure satisfactory electrical performance during discharge. The alkali metal-halogen electrochemical couple is typically constructed in a case-positive configuration with the case wall serving as the cathodic current collector. A contamination free inner surface for the casing facilitates electron transfer during discharge. It would, therefore, be highly desirable to provide the foregoing in a time-saving, economical and environmentally sound manner.